The present invention relates to a brake mechanism for suppressing the operation speed of an operation end such as a valve or damper provided with a return spring, and a powered actuator having this brake mechanism.
As a conventional powered actuator, a spring return type actuator having a valve or damper as the operation end is used. In this spring return type actuator, the rotation force of a drive motor is transmitted to the operation end through a reduction mechanism to increase the torque, thereby opening/closing the valve or damper constituting the operation end. The valve or damper is provided with a return spring. When power is not supplied to the drive motor because of a power failure or the like, the valve or damper is forcibly fully closed or opened with the force (restoration force) of the return spring. During the return operation of fully closing or opening the valve or damper forcibly, a braking force (brake torque) is effected to moderate impact generated during the full-closing or full-opening operation. An example of the braking method during this return operation includes the following and the like.
I. Inertia braking method employing weight PA1 II. Governor method utilizing friction PA1 III. Impeller method utilizing air resistance
I. Inertia Braking Method Employing Weight
With this method, as shown in FIG. 16A, a brake mechanism 1 having a disk 1-1 is rotationally connected midway along a power transmission line connected to an operation end. Weights 1-2A and 1-2B are arranged on the disk 1-1 and are connected to the rotation center through springs 1-3A and 1-3B. This increases the moment of inertia during the return operation and suppresses an increase in operation speed of the operation end. In this case, the moment of inertia can be changed by a centrifugal force, i.e., the rotation speed, and it is estimated that a brake torque T.sub.B be substantially constant with respect to a rotation speed N, as shown in FIG. 16B. Accordingly, the operation speed (return speed) of the operation end from the start of return to the end of return will show the characteristics as shown in FIG. 16C.
The characteristics shown in FIG. 16C are expressed by: EQU d.omega./dt=(TS-TB)/J
The moment J of inertia increases in proportion to the second power of the speed, and the brake torque T.sub.B is constant regardless of the rotation speed.
II. Governor Method Utilizing Friction
With this method, as shown in FIG. 17A, a brake mechanism 2 having a case 2-1 is rotationally connected midway along a power transmission line connected to an operation end. Drums 2-2A and 2-2B are arranged in the case 2-1 and are connected to the rotation center through springs 2-3A and 2-3B. During the return operation, the drums 2-2A and 2-2B are pulled in the radially outward direction by a centrifugal force to generate friction between them and the case 2-1, thereby suppressing an increase in operation speed of the operation end. In this case, it is estimated that a brake torque T.sub.B increase from a rotation speed N.sub.0, with which the drums 2-2A and 2-2B start to cause friction with the case 2-1, in substantially proportional to a rotation speed N, as shown in FIG. 17B. Accordingly, the return speed of the operation end from the start of return to the end of return will show the characteristics as shown in FIG. 17C.
The characteristics shown in FIG. 17C are expressed by: EQU d.omega./dt=(TS-TB)/J
The brake torque T.sub.B is constant when the rotation speed is equal to or smaller than a predetermined value, and is variable when the rotation speed exceeds the predetermined value.
III. Impeller Method Utilizing Air Resistance
With this method, as shown in FIG. 18A, a brake mechanism 3 having an impeller 3-1 is rotationally connected midway along a power transmission line connected to an operation end. During the return operation, the impeller 3-1 rotates to generate a braking force caused by an air resistance, thereby suppressing an increase in operation speed of the operation end. In this case, it is estimated that a brake torque T.sub.B increase in substantially proportional to a rotation speed N, as shown in FIG. 18B. Accordingly, the return speed of the operation end from the start of return to the end of return will show the characteristics as shown in FIG. 18C.
The characteristics shown in FIG. 18C are expressed by: EQU d.omega./dt=(TS-TB)/J
The brake torque T.sub.B can be changed in accordance with the rotation speed.
According to the conventional spring return type actuators of this type, however, since a braking force proportional to the rotation speed N cannot be obtained in the inertia braking method I, a stable operation speed cannot be obtained in the return operation of an operation end. Since the portion for generating the braking force causes friction in the governor method II, performance degradation occurs due to the friction. Since a braking force with respect to a shape is limited in the impeller method III, a large braking force cannot be obtained, and a large shape is required to obtain a large braking force. In any method, the structure is complicated to change the return time of the operation end.